This proposal is for a randomized double-blind placebo controlled clinical trial of the efficacy of nutritional supplementation with calcium carbonate in preventing recurrence of neoplastic polyps of the large bowel. This will be a collaborative study involving seven clinical centers and a coordinating center. Subjects will have had at least one neoplastic polyp removed in the three months before enrollment, with no known polyps remaining in the colon, and no contraindications to calcium supplementation. Eight hundred fifty subjects will be randomized; they will be allocated at random to receive either placebo or calcium carbonate, 3 grams per day (equivalent to 1200 mg elemental calcium). All subjects will be monitored for compliance and toxicity at six month intervals. One year and four years after the qualifying colonoscopy, each subject will recieve a complete colonoscopic examination, with removal and histological examination of all polyps. The primary analysis will be a comparison of the polyp recurrence rates in the placebo group with that in the calcium group. The study will have at least 80% power to detect a 35% reduction in polyp recurrence from a 10% per year recurrence rate. Secondary analysis will investigate the effects of treatment on the number, size, and histological atypia of polyps, as well as the relationship between baseline dietary calcium or other personal characteristics and polyp recurrence.